10
Detailed Description
The ISL84541–ISL84544 dual analog switches offer precise
switching capability from a single 2.7V to 12V supply with
low on-resistance (30) and high speed operation
(t
ON
= 35ns, t
OFF
= 25ns). The devices are especially well
suited to portable battery powered equipment thanks to the
low operating supply voltage (2.7V), low power consumption
(5W), low leakage currents (100pA max), and the tiny
SOT-23 packaging. High frequency applications also benefit
from the wide bandwidth, and the very high off isolation and
crosstalk rejection.
Supply Sequencing And Overvoltage Protection
With any CMOS device, proper power supply sequencing is
required to protect the device from excessive input currents
which might permanently damage the IC. All I/O pins contain
ESD protection diodes from the pin to V+ and to GND (see
Figure 8). To prevent forward biasing these diodes, V+ must
be applied before any input signals, and input signal voltages
must remain between V+ and GND. If these conditions cannot
be guaranteed, then one of the following two protection
methods should be employed.
Logic inputs can easily be protected by adding a 1k resistor
in series with the input (see Figure 8). The resistor limits the
input current below the threshold that produces permanent
damage, and the sub-microamp input current produces an
insignificant voltage drop during normal operation.
Adding a series resistor to the switch input defeats the
purpose of using a low R
ON
switch, so two small signal diodes
can be added in series with the supply pins to provide
overvoltage protection for all pins (see Figure 8). These
additional diodes limit the analog signal from 1V below V+ to
1V above GND. The low leakage current performance is
unaffected by this approach, but the switch resistance may
increase, especially at low supply voltages.
Power-Supply Considerations
The ISL8454X construction is typical of most CMOS analog
switches, except that they have only two supply pins: V+ and
GND. V+ and GND drive the internal CMOS switches and set
their analog voltage limits. Unlike switches with a 13V
maximum supply voltage, the ISL8454X 15V maximum supply
voltage provides plenty of room for the 10% tolerance of 12V
supplies, as well as room for overshoot and noise spikes.
The minimum recommended supply voltage is 2.7V. It is
important to note that the input signal range, switching times,
and on-resistance degrade at lower supply voltages. Refer to
the electrical specification tables and Typical Performance
curves for details.
V+ and GND also power the internal logic and level shifters.
The level shifters convert the logic levels to switched V+ and
GND signals to drive the analog switch gate terminals.
This family of switches cannot be operated with bipolar
supplies, because the input switching point becomes negative
in this configuration.
Logic-Level Thresholds
This switch family is TTL compatible (0.8V and 2.4V) over a
supply range of 3V to 11V (see Figure 15). At 12V the V
IH
level is about 2.5V. This is still below the TTL guaranteed
high output minimum level of 2.8V, but noise margin is
reduced. For best results with a 12V supply, use a logic
family the provides a V
OH
greater than 3V.
The digital input stages draw supply current whenever the
digital input voltage is not at one of the supply rails. Driving
the digital input signals from GND to V+ with a fast transition
time minimizes power dissipation.
High-Frequency Performance
In 5systems, signal response is reasonably flat even past
300MHz (see Figure 16). Figure 16 also illustrates that the
frequency response is very consistent over a wide V+ range,
and for varying analog signal levels.
An off switch acts like a capacitor and passes higher
frequencies with less attenuation, resulting in signal
feedthrough from a switch’s input to its output. Off Isolation is
the resistance to this feedthrough, while Crosstalk indicates
the amount of feedthrough from one switch to another.
Figure 17 details the high Off Isolation and Crosstalk
rejection provided by this family. At 10MHz, off isolation is
about 50dB in 5systems, decreasing approximately 20dB
per decade as frequency increases. Higher load
impedances decrease Off Isolation and Crosstalk rejection
due to the voltage divider action of the switch OFF
impedance and the load impedance.
Leakage Considerations
Reverse ESD protection diodes are internally connected
between each analog-signal pin and both V+ and GND.
One of these diodes conducts if any analog signal exceeds
V+ or GND.
Virtually all the analog leakage current comes from the ESD
diodes to V+ or GND. Although the ESD diodes on a given
signal pin are identical and therefore fairly well balanced,
GND
V
COM
V
NO or NC
OPTIONAL PROTECTION
V+
IN
X
DIODE
OPTIONAL PROTECTION
DIODE
OPTIONAL
PROTECTION
RESISTOR
FIGURE 8. OVERVOLTAGE PROTECTION
ISL84541, ISL84542, ISL84543, ISL84544
